undefined method `add_comment' for nil:NilClass (NoMethodError) ./tdiary/defaultio.rb:48:in `restore_comment' ./tdiary/defaultio.rb:39:in `each' ./tdiary/defaultio.rb:39:in `restore_comment' ./tdiary/defaultio.rb:37:in `open' ./tdiary/defaultio.rb:37:in `restore_comment' ./tdiary/defaultio.rb:154:in `transaction' ./tdiary.rb:1873:in `initialize' index.rb:52:in `new' index.rb:52